Method of manufacturing secondary emitting electrodes



1940- A. SUMMER 2,206,372

IETHOD OF MANUFACTURING SECONDARY EMITTING ELECTRODES Filed June 14, 1939 ATTDQN EY Patented July 2, 1940 UNITED STATES METHOD OF MANUFACTURING SECONDARY EMITTING ELECTRODES Alfred Summer, London, England, assignor to Baird Television Limited, London, England, a

British company Application June 14, 1939, Serial No. 279,039 In Great Britain March 15, 1939 5 Claims.

This invention relates to improvements in methods of manufacturing secondary emittin electrodes such as are used in electron multipliers and like electronic devices. In many forms of 5 electron device it is desirable that an electrode shall have as high a secondary emission ratio as possible and shall at the same time liave a low photoelectric sensitivity. For example, in one form of device for obtaining an image sig- 10 nal in a television or like system'photoelectrons liberated from a cathode under the influence of an optical image projected thereon are focussed upon a mosaic electrode, comprising a large number of elements having a comparatively high secondary emission ratio, to produce charges thereon. In this case it is obviously undesirable that the mosaic elements shall be photoelectrically sensitive in order to avoid spurious signals arising.

According to the present invention there is provided a method of manufacturing a secondary electron emitting electrode which comprises oxidising a known type of photoelectrically sensitive surface until the photoelectric sensitivity is reduced substantially to zero.

The present invention is particularly applicable to a photoelectrically sensitive surface of the type which comprises an alloy of an alkali metal with a non-alkali metal.

It isfound that if the process of oxidation required to sensitise the surface of this or other similar type of surface be carried past the stage at which maximum photoelectric sensitivity is 5 obtained until the photoelectric sensitivity is reduced to zero, a surface is produced having a high secondary emission ratio (of the order of 13) but having of course substantially no photoelectric sensitivity. Obviously the conditions 0 under which oxidation is carried out are varied to suit the type of photoelectric surface employed.

The invention will be hereinafter more particularly described with reference to the accompanying drawing comprising Figs. 1, 2 and 3 which illustrate in a purely schematic manner the stagesin the production of a secondary emitting electrode comprising an alloy of antimony and caesium. The process of manufacture is as 0 follows:

Referring to Fig. 1, a supporting member I, which may be a silver or glass plate is enclosed in a glass envelope which is then evacuated. A layer of antimony 2 is then deposited on the supporting member 1 by evaporation of a metal filament arranged to be heated by the passage of an electric current therethrough and having a pellet of antimony placed inside it. The electrode is then heated to a temperature in the region of 140 to 190 C. by enclosing the envelope 5 in an oven, and caesium is slowly evaporated on to the electrode from a side tube sealed into the envelope. The temperature at which the electrode is maintained is such that the caesium does not condense as a layer on the electrode 19 but diffuses into the body of the layer. The alloy of antimony and caesium which is thus formed is indicated by the reference numeral 3 in Fig. 2. When the observed photosensitivity slightly exceeds the maximum the introduction 15 of caesium is stopped and the side tube sealed off from the envelope. The electrode is then heated at about 200 C. until the observed photosensitivity no longer increases. The electrode is then cooled to room temperature and oxygen 80 is introduced into the envelope at a very slow rate. The photosensitivity is observed and rises at first to a maximum after which it falls slowly to zero.

Fig. 3 illustrates purely schematically the final ll form of the electrode comprising the supporting member I, the layer of antimony and caesium alloy 3 and the oxygen layer 4. It is to be understood that the layer 4 will-not comprise a separate layer of oxygen located on the surface of the 80 layer 3 but in actual fact the oxygen will be bound by molecular forces to the surface of the layer 3. When the photoelectric current is no longer observable the introduction of oxygen is stopped and the vessel completely evacuated. It

The use of electrodes manufactured in accordance with the present invention is particularly advantageous in the case of electron multipliers for increasing the output current from a phototube in a television system or for measurement 0 purposes in which it is undesirable that stray light striking the multiplier electrodes shall cause photo emission. To construct a phototube containing an electron multiplier sensitised by this process it is necessary to divide the tube into two 5 portions during manufacture by a partition which may be destroyed by the impact of a mobile object after sensitisation of the photocathode and multiplier electrodes has been completed.

I claim: 50

1. The method of manufacturing a secondary electron emissive electrode which comprises oxidising an alloy of antimony and caesium untfl the photoelectric sensitivity is reduced substantially to zero. ll

rating caesium onto said layer while heating said layer to a temperature such that said caesium diil'uses into said layer, continuing the evaporation of caesium until the observed photoelectric emission from said layer has reached a maximum, heating the composite layer to a higher temperature, cooling the composite layer, admitting oxygen into said envelope until the photoelectric emission !rom said composite layer is reduced substantially to zero and again evacuating said envelope.

4. The method 01' manufacturing a secondary electron emissive electrode which comprises enclosing a supporting member in an envelope, evacuating said envelope, evaporating a layer of antimony onto said supporting member, heating said envelope so that caesium vapour is introduced thereinto from a side tube and dif- Iuses into said layer to form an alloy with' said antimony and until the observed photosensitivity of the alloy layer has reached a maximum, sealing ofl said side tube from said envelope, heating said electrode to a higher temperature until the photoelectric sensitivity is a maximum, cooling said "electrode, slowly admitting oxygen into said envelope until the photoelectric sensitivity is reduced substantially to zero and again 

